Liquid crystals are used in a wide variety of devices, including visual display devices. The property of liquid crystals that enables them to be used, for example, in visual displays, is the ability of liquid crystals to transmit light on one hand, and to scatter light and/or to absorb it (especially when combined with an appropriate dye), on the other, depending on whether the liquid crystals are in a relatively free, that is de-energized or field-off state, or in a strictly aligned, that is energized or field-on state. An electric field selectively applied across the liquid crystals can be used to switch between field-off and field-on states.
There are three categories of liquid crystal materials, namely, cholesteric, nematic and smectic. The present invention relates in the preferred embodiment described hereinafter to the use of liquid crystal material which is operationally nematic. By "operationally nematic" is meant that, in the absence of external fields, structural distortion of the liquid crystal is dominated by the orientation of the liquid crystal and its boundaries rather than by bulk effects, such as very strong twists (as in cholesteric material) or layering (as in smectic material). Thus, for example, a liquid crystal material including chiral ingredients which induce a tendency to twist but which cannot overcome the effects of the boundary alignment of the liquid crystal material would be considered to be operationally nematic. A more detailed explanation of operationally nematic liquid crystal material is provided in co-pending U.S. patent application Ser. No. 477,242, filed Mar. 21, 1983, in the name of Fergason, entitled ENCAPSULATED LIQUID CRYSTAL AND METHOD, assigned to Manchester R&D Partnership, the disclosure of which is hereby incorporated by reference. Reference may also be made to co-pending U.S. patent application Ser. No. 302,780, filed Sept. 16, 1981, in the name of Fergason, entitled ENCAPSULATED LIQUID CRYSTAL AND METHOD, assigned to Manchester R&D Partnership, which disclosure is also hereby incorporated by reference.
However, it is to be understood that the various principles of the present invention may be employed with any of the various types of liquid crystal materials or combinations thereof, including combinations with dyes. Designation of the apparatus of the present invention as a NCAP liquid crystal apparatus or reference to NCAP liquid crystal is in no way intended to limit such apparatus to use with nematic liquid crystal materials. It is only for convenience sake and in an effort to use an abbreviated term that describes the present invention that reference is made to NCAP liquid crystal. Particularly, the term NCAP is used because the preferred liquid crystal material is nematic or operationally nematic liquid crystal and because in the field-off condition, or any other condition which results in the liquid crystal being in a distorted or randomly aligned state, the liquid crystal structure is distorted to a curved form (hence curvilinearly aligned) wherein the spatial average orientation of the liquid crystal material over a capsule-like volume, for instance, is strongly curved and there is no substantial parallel directional orientation of the liquid crystal in the absence of a prescribed input, for example, an electric field.
The present invention is particularly adapted for use with an apparatus using NCAP liquid crystal. NCAP liquid crystals and the method of making the same and devices using NCAP liquid crystals are described in detail in the above-identified U.S. patent application Ser. No. 302,780. Briefly, NCAP liquid crystal comprises a liquid crystal material that is more or less confined or contained in an encapsulating medium. A NCAP liquid crystal apparatus is an apparatus formed of NCAP liquid crystals that are capable of providing a function of the type typically inuring to a liquid crystal material. For example, such a NCAP liquid crystal apparatus may be a visual display device that responds to the application and removal of an electric field to effect a selected attenuation of visible light.
Such NCAP liquid crystal apparatus may commonly include NCAP liquid crystals positioned between two electrodes wherein the electrodes are located on respective substrates. Highly conductive, non-transparent electrical paths may connect the respective electrodes to a source of electrical power via a selectively closable switch or switches so that the NCAP liquid crystal material may be switched between a field-on and field-off state. In visual display devices, for reasons of aesthetics and to facilitate viewing of data, it is desirable that the conductive electrical paths be more or less invisible to one observing the display. For example, in a direct-driven, dot matrix-addressed display comprising numerous pixel elements, an electrical lead or conductive path has to be brought to each dot or pixel. The presence of so many leads in the viewing area of such a display would obviously detract from its appearance and interfere with a viewer's ability to read the display.